1. Field of the Invention
The present invention relates to a system for improving the accuracy of non-destructive and non-intrusive testing techniques of moving web-like materials, such as paper, paperboard and other porous materials produced by the paper industry and, in particular, a system for improving the measurement of the velocity of ultrasonic signals in the plane of a moving web.
2. Description of the Prior Art
Web-like materials, such as paper, paperboard and the like are required to meet particular mechanical property specifications. Normal quality control techniques require that the web-like materials be tested to insure that the web uniformly meets the desired mechanical property specifications.
Destructive-type tests are known for measuring mechanical properties of such web-like materials. Such destructive tests are normally conducted off-line on representative samples of such web-like materials. There are various problems with such off-line destructive testing techniques. For example, such testing is relatively time consuming and requires production to be stopped or sampled periodically when product is received from the machine. In addition, since such testing is destructive, it is normally performed on representative samples of the web which may be taken, for example, every several thousand square feet of material. In such a situation, a substantial amount of waste is incurred if the web-like material is found to fail the test.
In order to solve the problems associated with such destructive test-type measurements of mechanical properties of web-like materials, ultrasonic testing techniques have been developed. Such testing is done on-line and thus is relatively quicker than off-line destructive testing, while at the same time provides a relatively continuous indication of various mechanical properties of the web-like material to assure virtually uniform quality of the product while minimizing waste.
Both in-plane and out-of-plane ultrasonic testing techniques are known for moving web-like materials. Out- of-plane ultrasonic testing relates to the measurement of the velocity of an ultrasonic signal through the thickness of the moving web-like material in order to determine various mechanical properties of the web. In such out-of-plane ultrasonic velocity measurement systems, ultrasonic transducers are normally disposed on opposing sides of the moving web. The velocity of the ultrasonic signal through the thickness of the web is measured in order to determine the mechanical properties of interest.
The velocity of an ultrasonic signal in the plane of the web is useful for determining certain mechanical properties of the web. Various systems are known for measuring the velocity of an ultrasonic signal in the plane of the web. In one system, such as disclosed in U.S. Pat. No. 4,730,492, a plurality of ultrasonic transducers are disposed along an outer surface of a hollow cylinder. As the transducers contact the web, the system measures the velocity of an ultrasonic signal in the plane of the moving web in both a longitudinal direction and a transverse direction relative to the web.
Although such systems may provide adequate measurement of ultrasonic signals in the plane of the web, there are problems with such systems. One problem with such a system relates to the variability of the contact force between the transducers and the web. Since the contact force between the transducers and the web affects the acoustical coupling therebetween, it is desirable that the contact force be relatively constant. However, various factors are known to affect the contact force including the manner in which the transducers are mounted to a rotating wheel or cylinder that carries the transducers. In known systems, the transducers are rigidly mounted relative to the cylinder or wheel. With such a system, adjustment of the transducers relative to the wheel or cylinder in order to adjust the contact force has been relatively difficult, often requiring the removal of the wheel or cylinder, making the adjustment of the transducers relatively difficult and time consuming.
Another problem with such a system is that the cylinder or wheels are known to depend upon friction to move at the speed of the web. As such, the web is partially wrapped about the cylinder or wheel. There are several problems with such friction-driven systems. For example, in such a system, an inertial load is placed on the web. As such, it would not be usable for certain products, such as relatively thin papers and tissues. Additionally, such friction-driven systems are known to cause undue wear to the transducers resulting from web to transducer speed differences. Moreover, ultrasonic signals in such a system may simultaneously propagate through the cylinder resulting in unpredictable interference at the ultrasonic receivers causing errors in the velocity measurements.
Moreover, such systems as described above are generally not suitable for use with relatively wide webs (e.g., webs wider than the length of the cylinder). In such applications, it is known to use a system, for example, as disclosed in U.S. Pat. No. 4,291,577. In that system, a pair of friction-driven, axially aligned wheels are disposed on one side of a moving web. Transducers disposed in the peripheral surface of the wheels enable the velocity of ultrasonic signals in the plane of the web to be measured. However, with such a system, it is relatively difficult to maintain good contact between the transducers and the web since the web tension decreases near the edges.
There are other problems with such in-plane ultrasonic testing systems. More particularly, known in-plane ultrasonic testing systems are adapted to provide an on-line indication of various properties of a moving web which depend on the velocity of ultrasonic signals in a machine direction (e.g., MD, the direction of travel of the moving web) and a cross-direction (e.g., CD, perpendicular to the machine direction). However, some useful mechanical properties of web-like materials depend on velocity measurements in directions other than the MD and the CD. For example, in-plane polar specific stiffness measurements require velocity measurements along successive axes, 5.degree.-10.degree. apart. As such, such measurements must be made off-line.